Hydrophilic polymers for use in membrane electrode assemblies are disclosed in WO2003/023890. That publication describes how hydrophilic polymers can be obtained from the polymerisation of a hydrophobic monomer, a hydrophilic monomer, a monomer including a strongly ionic group and water.
The polymers disclosed in WO2003/023890 have excellent ionic properties, and are less susceptible to dehydration, in use, than non-hydrophilic membranes such as Nafion®. That publication describes, as a preferred embodiment, that the hydrophilic polymer should have a water uptake of no more than 85%. This is equivalent to a water content of approximately 35%. This is because it was believed that polymers with a higher water uptake than this, would be too unstable for use in an MEA. Water uptake is defined therein as [(hydrated mass−dry mass)/dry mass]×100.
The Examples in WO2003/023890 disclose MEAs (Membrane Electrode Assemblies), wherein the polymer has been further expanded by soaking in water. Due to the composition of those polymers, and also because they are polymerised onto a reinforcing material none of the final polymers in the MEAs have a water content of greater than 60%.
The water uptake of a hydrophilic polymer membrane is often restricted by polymerising the membrane onto a porous support structure, such as polythene. This prohibits the polymer from expanding too much, and therefore becoming too unstable. It also ensures that the anionic/cationic sites are dense, i.e. that the membrane has an intrinsically high acid site concentration, [H+]. This was believed to be necessary for the ionic performance of the membrane.